Efficient MIMO transmission schemes

ABSTRACT

A method for communication includes, in a transmitter having a first number of transmit antenna ports, setting an upper limit on a second number of spatial layers to be used by the transmitter to be less than the first number. An actual number of the spatial layers, which does not exceed the upper limit, is allocated for transmission to a given receiver. One or more streams of modulated symbols are mapped onto the allocated actual number of the spatial layers. The actual number of the spatial layers are transmitted from the transmitter to the given receiver.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.12/477,152, filed Jun. 3, 2009, which claims the benefit of U.S.Provisional Patent Application 61/142,735, filed Jan. 6, 2009, and U.S.Provisional Patent Application 61/175,197, filed May 4, 2009. Thedisclosures of all these related applications are incorporated herein byreference.

FIELD OF THE INVENTION

The present invention relates generally to communication systems, andparticularly to methods and systems for transmission using multipleantennas.

BACKGROUND OF THE INVENTION

Some communication systems transmit data from a transmitter to areceiver over multiple communication channels, using multiple transmitantennas and multiple receive antennas. Multiple-channel transmission isused, for example, in spatial multiplexing schemes that achieve highthroughput, in beam-forming schemes that achieve high antennadirectivity and in spatial diversity schemes that achieve highresilience against channel fading and multipath. These schemes are oftenreferred to collectively as Multiple-Input Multiple-Output (MIMO)schemes.

MIMO schemes are contemplated, for example, for use in Evolved UniversalTerrestrial Radio Access (E-UTRA) systems, also referred to as Long TermEvolution (LTE) systems. The Third Generation Partnership Project (3GPP)E-UTRA standards specify MIMO schemes for use by E-UTRA User Equipment(UE) and base stations (eNodeB). These schemes are described, forexample, in 3GPP Technical Specification 36.211, entitled “TechnicalSpecification Group Radio Access Network; Evolved Universal TerrestrialRadio Access (E-UTRA); Physical Channels and Modulation (Release 8),”(3GPP TS 36.211), version 8.6.0, March, 2009, in 3GPP TechnicalSpecification 36.213, entitled “Technical Specification Group RadioAccess Network; Evolved Universal Terrestrial Radio Access (E-UTRA);Physical Layer Procedures (Release 8),” (3GPP TS 36.213), version 8.6.0,March, 2009, and in 3GPP Technical Report 36.814, entitled “TechnicalSpecification Group Radio Access Network; Further Advancements forE-UTRA Physical Layer Aspects (Release 9),” (3GPP TR 36.814), version0.4.1, February, 2009, which are incorporated herein by reference.

SUMMARY OF THE INVENTION

An embodiment of the present invention provides a method forcommunication in a transmitter having a first number of transmit antennaports. In accordance with the disclosed method, an upper limit is set ona second number of spatial layers to be used by the transmitter to beless than the first number. An actual number of the spatial layers,which does not exceed the upper limit, is allocated for transmission toa given receiver. One or more streams of modulated symbols are mappedonto the allocated actual number of the spatial layers. The actualnumber of the spatial layers is transmitted from the transmitter to thegiven receiver.

In some embodiments, transmitting the spatial layers includes applying aprecoding operation that maps the spatial layers onto the transmitantenna ports, and setting the upper limit includes setting a firstupper limit when the precoding operation depends on feedback from thegiven receiver, and setting a second upper limit, which does not exceedthe first upper limit, when the precoding operation is not dependent onthe feedback.

In an embodiment, input data is encoded with an Error Correction Code(ECC) to produce a given number of code words, and the code words aremodulated to produce the respective given number of the streams of themodulated symbols, wherein the given number of the code words isrestricted to be at most two.

In a disclosed embodiment, the modulated symbols are mapped onto thelayers in accordance with the following table, in which d^((q))(n)denotes an n^(th) modulated symbol originating from a code word q, andx^((p))(n) denotes an n^(th) value of a spatial layer p:

Number Number of of code Codeword-to-layer mapping layers words i = 0,1, . . . , M_(symb) ^(layer) − 1 1 1 x⁽⁰⁾(i) = d⁽⁰⁾(i) M_(symb) ^(layer)= M_(symb) ⁽⁰⁾ 2 2 x⁽⁰⁾(i) = d⁽⁰⁾(i) M_(symb) ^(layer) = M_(symb) ⁽⁰⁾ =M_(symb) ⁽¹⁾ x⁽¹⁾(i) = d⁽¹⁾(i) 2 1 x⁽⁰⁾(i) = d⁽⁰⁾(2i) M_(symb) ^(layer)= M_(symb) ⁽⁰⁾/2 x⁽¹⁾(i) = d⁽⁰⁾(2i + 1) 3 2 x⁽⁰⁾(i) = d⁽⁰⁾(i) M_(symb)^(layer) = M_(symb) ⁽⁰⁾ = x⁽¹⁾(i) = d⁽¹⁾(2i) M_(symb) ⁽¹⁾/2 x⁽²⁾(i) =d⁽¹⁾(2i + 1) 3 1 x⁽⁰⁾(i) = d⁽⁰⁾(3i) M_(symb) ^(layer) = M_(symb) ⁽⁰⁾/3x⁽¹⁾(i) = d⁽⁰⁾(3i + 1) x⁽²⁾(i) = d⁽⁰⁾(3i + 2) 4 2 x⁽⁰⁾(i) = d⁽⁰⁾(2i)M_(symb) ^(layer) = M_(symb) ⁽⁰⁾/2 = x⁽¹⁾(i) = d⁽⁰⁾(2i + 1) M_(symb)⁽¹⁾/2 x⁽²⁾(i) = d⁽¹⁾(2i) x⁽³⁾(i) = d⁽¹⁾(2i + 1) 4 1 x⁽⁰⁾(i) = d⁽⁰⁾(4i)M_(symb) ^(layer) = M_(symb) ⁽⁰⁾/4 x⁽¹⁾(i) = d⁽⁰⁾(4i + 1) x⁽²⁾(i) =d⁽⁰⁾(4i + 2) x⁽³⁾(i) = d⁽⁰⁾(4i + 3)

In another embodiment, the modulated symbols are mapped onto the layersin accordance with the following table:

5 2 x⁽⁰⁾(i) = d⁽⁰⁾(2i) M_(symb) ^(layer) = M_(symb) ⁽⁰⁾/2 = M_(symb)⁽¹⁾/3 x⁽¹⁾(i) = d⁽⁰⁾(2i + 1) x⁽²⁾(i) = d⁽¹⁾(3i) x⁽³⁾(i) = d⁽¹⁾(3i + 1)x⁽⁴⁾(i) = d⁽¹⁾(3i + 2) 6 2 x⁽⁰⁾(i) = d⁽⁰⁾(3i) M_(symb) ^(layer) =M_(symb) ⁽⁰⁾/3 = M_(symb) ⁽¹⁾/3 x⁽¹⁾(i) = d⁽⁰⁾(3i + 1) x⁽²⁾(i) =d⁽⁰⁾(3i + 2) x⁽³⁾(i) = d⁽¹⁾(3i) x⁽⁴⁾(i) = d⁽¹⁾(3i + 1) x⁽⁵⁾(i) = d⁽¹⁾(3i− 2) 7 2 x⁽⁰⁾(i) = d⁽⁰⁾(3i) M_(symb) ^(layer) = M_(symb) ⁽⁰⁾/3 =M_(symb) ⁽¹⁾/4 x⁽¹⁾(i) = d⁽⁰⁾(3i + 1) x⁽²⁾(i) = d⁽⁰⁾(3i + 2) x⁽³⁾(i) =d⁽¹⁾(4i) x⁽⁴⁾(i) = d⁽¹⁾(4i + 1) x⁽⁵⁾(i) = d⁽¹⁾(4i + 2) x⁽⁶⁾(i) =d⁽¹⁾(4i + 3) 8 2 x⁽⁰⁾(i) = d⁽⁰⁾(4i) M_(symb) ^(layer) = M_(symb) ⁽⁰⁾/4 =M_(symb) ⁽¹⁾/4 x⁽¹⁾(i) = d⁽⁰⁾(4i + 1) x⁽²⁾(i) = d⁽⁰⁾(4i + 2) x⁽³⁾(i) =d⁽⁰⁾(4i + 3) x⁽⁴⁾(i) = d⁽¹⁾(4i) x⁽⁵⁾(i) = d⁽¹⁾(4i + 1) x⁽⁶⁾(i) =d⁽¹⁾(4i + 2) x⁽⁷⁾(i) = d⁽¹⁾(4i + 3)

In yet another embodiment, the modulated symbols are mapped onto thelayers in accordance with the following table:

5 2 x⁽⁰⁾(i) = d⁽⁰⁾(3i) M_(symb) ^(layer) = M_(symb) ⁽⁰⁾/3 = M_(symb)⁽¹⁾/2 x⁽¹⁾(i) = d⁽⁰⁾(3i + 1) x⁽²⁾(i) = d⁽⁰⁾(3i + 2) x⁽³⁾(i) = d⁽¹⁾(2i)x⁽⁴⁾(i) = d⁽¹⁾(2i + 1) 6 2 x⁽⁰⁾(i) = d⁽⁰⁾(3i) M_(symb) ^(layer) =M_(symb) ⁽⁰⁾/3 = M_(symb) ⁽¹⁾/3 x⁽¹⁾(i) = d⁽⁰⁾(3i + 1) x⁽²⁾(i) =d⁽⁰⁾(3i + 2) x⁽³⁾(i) = d⁽¹⁾(3i) x⁽⁴⁾(i) = d⁽¹⁾(3i + 1) x⁽⁵⁾(i) =d⁽¹⁾(3i + 2) 7 2 x⁽⁰⁾(i) = d⁽⁰⁾(4i) M_(symb) ^(layer) = M_(symb) ⁽⁰⁾/4 =M_(symb) ⁽¹⁾/3 x⁽¹⁾(i) = d⁽⁰⁾(4i + 1) x⁽²⁾(i) = d⁽⁰⁾(4i + 2) x⁽³⁾(i) =d⁽⁰⁾(4i + 2) x⁽⁴⁾(i) = d⁽¹⁾(3i) x⁽⁵⁾(i) = d⁽¹⁾(3i + 1) x⁽⁶⁾(i) =d⁽¹⁾(3i + 2) 8 2 x⁽⁰⁾(i) = d⁽⁰⁾(4i) M_(symb) ^(layer) = M_(symb) ⁽⁰⁾/4 =M_(symb) ⁽¹⁾/4 x⁽¹⁾(i) = d⁽⁰⁾(4i + 1) x⁽²⁾(i) = d⁽⁰⁾(4i + 2) x⁽³⁾(i) =d⁽⁰⁾(4i + 3) x⁽⁴⁾(i) = d⁽¹⁾(4i) x⁽⁵⁾(i) = d⁽¹⁾(4i + 1) x⁽⁶⁾(i) =d⁽¹⁾(4i + 2) x⁽⁷⁾(i) = d⁽¹⁾(4i + 3)

In still another embodiment, the modulated symbols are mapped onto thelayers in accordance with the following table:

5 2 x⁽⁰⁾(i) = d⁽⁰⁾(2i) M_(symb) ^(layer) = M_(symb) ⁽⁰⁾/2 = M_(symb)⁽¹⁾/3 x⁽¹⁾(i) = d⁽⁰⁾(2i + 1) x⁽²⁾(i) = d⁽¹⁾(3i) x⁽³⁾(i) = d⁽¹⁾(3i + 1)x⁽⁴⁾(i) = d⁽¹⁾(3i + 2) 6 2 x⁽⁰⁾(i) = d⁽⁰⁾(3i) M_(symb) ^(layer) =M_(symb) ⁽⁰⁾/3 = M_(symb) ⁽¹⁾/3 x⁽¹⁾(i) = d⁽⁰⁾(3i + 1) x⁽²⁾(i) =d⁽⁰⁾(3i + 2) x⁽³⁾(i) = d⁽¹⁾(3i) x⁽⁴⁾(i) = d⁽¹⁾(3i + 1) x⁽⁵⁾(i) =d⁽¹⁾(3i + 2) 7 2 x⁽⁰⁾(i) = d⁽⁰⁾(4i) M_(symb) ^(layer) = M_(symb) ⁽⁰⁾/4 =M_(symb) ⁽¹⁾/3 x⁽¹⁾(i) = d⁽⁰⁾(4i + 1) x⁽²⁾(i) = d⁽⁰⁾(4i + 2) x⁽³⁾(i) =d⁽⁰⁾(4i + 2) x⁽⁴⁾(i) = d⁽¹⁾(3i) x⁽⁵⁾(i) = d⁽¹⁾(3i + 1) x⁽⁶⁾(i) =d⁽¹⁾(3i + 2) 8 2 x⁽⁰⁾(i) = d⁽⁰⁾(4i) M_(symb) ^(layer) = M_(symb) ⁽⁰⁾/4 =M_(symb) ⁽¹⁾/4 x⁽¹⁾(i) = d⁽⁰⁾(4i + 1) x⁽²⁾(i) = d⁽⁰⁾(4i + 2) x⁽³⁾(i) =d⁽⁰⁾(4i + 3) x⁽⁴⁾(i) = d⁽¹⁾(4i) x⁽⁵⁾(i) = d⁽¹⁾(4i + 1) x⁽⁶⁾(i) =d⁽¹⁾(4i + 2) x⁽⁷⁾(i) = d⁽¹⁾(4i + 3)

In another embodiment, the modulated symbols are mapped onto the layersin accordance with the following table:

5 2 x⁽⁰⁾(i) = d⁽⁰⁾(3i) M_(symb) ^(layer) = M_(symb) ⁽⁰⁾/3 = M_(symb)⁽¹⁾/2 x⁽¹⁾(i) = d⁽⁰⁾(3i + 1) x⁽²⁾(i) = d⁽⁰⁾(3i + 2) x⁽³⁾(i) = d⁽¹⁾(2i)x⁽⁴⁾(i) = d⁽¹⁾(2i + 1) 6 2 x⁽⁰⁾(i) = d⁽⁰⁾(3i) M_(symb) ^(layer) =M_(symb) ⁽⁰⁾/3 = M_(symb) ⁽¹⁾/3 x⁽¹⁾(i) = d⁽⁰⁾(3i + 1) x⁽²⁾(i) =d⁽⁰⁾(3i + 2) x⁽³⁾(i) = d⁽¹⁾(3i) x⁽⁴⁾(i) = d⁽¹⁾(3i + 1) x⁽⁵⁾(i) =d⁽¹⁾(3i + 2) 7 2 x⁽⁰⁾(i) = d⁽⁰⁾(3i) M_(symb) ^(layer) = M_(symb) ⁽⁰⁾/3 =M_(symb) ⁽¹⁾/4 x⁽¹⁾(i) = d⁽⁰⁾(3i + 1) x⁽²⁾(i) = d⁽⁰⁾(3i + 2) x⁽³⁾(i) =d⁽¹⁾(4i) x⁽⁴⁾(i) = d⁽¹⁾(4i + 1) x⁽⁵⁾(i) = d⁽¹⁾(4i + 2) x⁽⁶⁾(i) =d⁽¹⁾(4i + 3) 8 2 x⁽⁰⁾(i) = d⁽⁰⁾(4i) M_(symb) ^(layer) = M_(symb) ⁽⁰⁾/4 =M_(symb) ⁽¹⁾/4 x⁽¹⁾(i) = d⁽⁰⁾(4i + 1) x⁽²⁾(i) = d⁽⁰⁾(4i + 2) x⁽³⁾(i) =d⁽⁰⁾(4i + 3) x⁽⁴⁾(i) = d⁽¹⁾(4i) x⁽⁵⁾(i) = d⁽¹⁾(4i + 1) x⁽⁶⁾(i) =d⁽¹⁾(4i + 2) x⁽⁷⁾(i) = d⁽¹⁾(4i + 3)

In some embodiments, the first number is greater than four. In anembodiment, the first number is equal to eight, and the upper limit isbetween four and seven.

In an embodiment, allocating the actual number of the spatial layersincludes signaling the actual number to the given receiver using asignaling protocol in which a data structure allocated to signaling theactual number is insufficient for signaling values higher than the upperlimit. Additionally or alternatively, allocating the actual number ofthe spatial layers may include signaling from the given receiver to thetransmitter a preferred number of the spatial layers using a signalingprotocol in which a data structure allocated to signaling the preferrednumber is insufficient for signaling values higher than the upper limit.In a disclosed embodiment, the spatial layers include respectiveparallel streams transmitted concurrently from the transmitter to thegiven receiver.

There is additionally provided, in accordance with an embodiment of thepresent invention, a method for communication in a transmitter having aplurality of transmit antenna ports and is operative to map streams ofmodulated symbols onto spatial layers. In accordance with the disclosedmethod, a precoding operation is selected for use in mapping the spatiallayers onto the transmit antenna ports. An upper limit is set on anumber of the spatial layers depending on the selected precodingoperation. One or more of the streams of the modulated symbols aremapped onto the number of spatial layers that does not exceed the upperlimit. The selected precoding operation is applied to the spatial layersso as to map the spatial layers onto the transmit antenna ports. Theprecoded spatial layers are transmitted over the transmit antenna portsto a receiver.

In some embodiments, setting the upper limit includes setting a firstupper limit when the selected precoding operation depends on feedbackfrom the receiver, and setting a second upper limit, which is less thanthe first upper limit, when the selected precoding operation is notdependent on the feedback. In an embodiment, input data is encoded withan Error Correction Code (ECC) to produce a given number of code words,and the code words are modulated to produce the streams of the modulatedsymbols, wherein the given number of the code words is restricted to beat most two. In a disclosed embodiment, mapping the streams onto thespatial layers includes signaling the number of the spatial layers tothe receiver using a signaling protocol in which a data structureallocated to signaling the number of the spatial layers is insufficientfor signaling values higher than the upper limit. Additionally oralternatively, mapping the streams onto the spatial layers includessignaling from the receiver to the transmitter a preferred number of thespatial layers using a signaling protocol in which a data structureallocated to signaling the preferred number is insufficient forsignaling values higher than the upper limit. In some embodiments, thespatial layers include respective parallel streams transmittedconcurrently from the transmitter to the receiver.

There is also provided, in accordance with an embodiment of the presentinvention, a communication apparatus, which includes a transmitter and afirst number of transmit antenna ports. The transmitter is configured toset an upper limit on a second number of spatial layers to be used bythe transmitter to be less than the first number, to allocate an actualnumber of the spatial layers, which does not exceed the upper limit, fortransmission to a given receiver, to map one or more streams ofmodulated symbols onto the allocated actual number of the spatiallayers, and to transmit the actual number of the spatial layerssimultaneously to the given receiver. The transmitter may be included ina mobile communication terminal or in a base station.

There is further provided, in accordance with an embodiment of thepresent invention, a communication apparatus, which includes atransmitter and a first number of transmit antenna ports. Thetransmitter is configured to map streams of modulated symbols ontospatial layers, to select a precoding operation for use in mapping thespatial layers onto the transmit antenna ports, to set an upper limit ona number of the spatial layers depending on the selected precodingoperation, to map one or more of the streams of the modulated symbolsonto the number of spatial layers that does not exceed the upper limit,to apply the selected precoding operation to the spatial layers so as tomap the spatial layers onto the transmit antenna ports, and to transmitthe precoded spatial layers over the transmit antenna ports to areceiver. The transmitter may be included in a mobile communicationterminal or in a base station.

The present invention will be more fully understood from the followingdetailed description of the embodiments thereof, taken together with thedrawings in which:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram that schematically illustrates a transmitterhaving multiple antennas, in accordance with an embodiment of thepresent invention;

FIGS. 2 and 3 are flow charts that schematically illustrate methods fortransmission via multiple antennas, in accordance with embodiments ofthe present invention; and

FIGS. 4A-7B are tables showing mapping of code words to spatial layers,in accordance with embodiments of the present invention.

DETAILED DESCRIPTION OF EMBODIMENTS

In some known MIMO schemes, a transmitter maps streams of modulatedsymbols onto spatial layers, i.e., signals that are to be transmittedover different MIMO transmission channels. The spatial layers are alsoreferred to as transmission layers or spatial streams, or simply layersfor brevity. The transmitter then applies a precoding operation to mapeach spatial layer onto a respective set of antenna ports. Atransmission process of this sort, as performed in the downlink of aE-UTRA eNodeB, is described in detail in section 6.3 of the 3GPP TS36.211 specification, cited above.

Embodiments of the present invention that are described hereinbelowprovide improved transmitter configurations and transmission methods,which reduce the complexity of MIMO transmitters and simplify theabove-mentioned transmission process and associated signaling.

Generally, the number of spatial layers that are actually used fortransmission from a given transmitter to a given receiver may be variedaccording to channel conditions. In conventional E-UTRA systems, forexample, the number of spatial layers may reach min (N_(TX), N_(RX))wherein N_(TX) and N_(RX) denote the number of transmit and receiveantenna ports, respectively. In many cases, however, it may beadvantageous to use an even lower number of spatial layers.

Typically, the actual number of layers that can be used depends on thelevel of correlation among the different communication channels betweenthe transmitter and the receiver (i.e., between different transmit andreceive antenna pairs). Low correlation usually implies that a largenumber of parallel transmission streams can be transmitted andreconstructed successfully, meaning that a large number of layers can beused. High correlation usually means that the number of layers should besmall.

In most cases, at least some correlation exists among the multiplecommunication channels, and therefore the likelihood of exploiting themaximum theoretical number of layers is small. Consequently, in mostpractical scenarios, it is sufficient to limit the number of actuallayers to a value that is less than the maximum theoretical limit of min(N_(TX), N_(RX)). Thus, in some embodiments of the present invention,the transmitter limits the number of layers to be less than the numberof transmit antennas. This limit is typically set a-priori, irrespectiveof channel conditions or the number of receive antennas in any givenreceiver. Limiting the maximum number of layers reduces the complexityof the transmitter considerably. The benefit of this technique isparticularly significant in evolving LTE-Advanced (LTE-A) systems, whichmay use up to eight transmit antennas and eight receive antennas.

In some embodiments, the transmitter sets different upper limits on thenumber of layers depending on feedback from the receiver and/or the typeof precoding operation used. For example, when precoding is adaptivebased on feedback from the receiver, in other words when precoding isperformed in closed loop, the ability to exploit the spatialmultiplexing gain of the multiple channels is relatively high, andtherefore the transmitter may allow a higher maximum number of layers.On the other hand, when precoding is performed in open loop, i.e.,without feedback from the receiver, the transmitter may set a lowerlimit on the maximum number of layers.

FIG. 1 is a block diagram that schematically illustrates a transmitter20 having multiple antennas, in accordance with an embodiment of thepresent invention. The description that follows refers to a transmitterof an LTE-A eNodeB, although other transmitters are contemplated. Inalternative embodiments, for example, the methods and systems describedherein can be used in transmitters operating in accordance with anyother suitable communication standard or protocol, such as IEEE 802.16(also referred to as WiMAX), for example. Although the description thatfollows refers mainly to downlink transmission from the eNodeB to theUE, the disclosed methods and systems may be applicable to uplinktransmission, as well.

Transmitter 20 comprises one or more modulation chains, each comprisingan Error Correction Code (ECC) encoder 24, a scrambler 28 and amodulation mapper 32. Data for transmission is encoded by ECC encoders24, to produce respective ECC code words. (The example of FIG. 1 showstwo separate ECC encoders for clarity. In practice, however, thetransmitter may comprise a single ECC encoder that produces code wordsfor the different modulation chains.) The number of code words that areused for encoding a given transmission is referred to as N_(CW). Certainaspects regarding the choice of this value are addressed further below.

The bits of each code word are scrambled by a respective scrambler 28,and then modulated by a respective modulation mapper 32. Each modulationmapper produces a stream of complex-valued modulated symbols. Anysuitable modulation scheme, such as Quadrature Phase Shift Keying (QPSK)or Quadrature Amplitude Modulation (QAM), can be used. A givenmodulation mapper 32 operates on the scrambled bits of a given code worddenoted q (g=0, 1, . . . , N_(CW)−1) to produce a block of M_(symb)^((q)) complex-valued modulated symbols denoted d^((q))(0), d^((q))(1),. . . , d^((q))(M_(symb) ^((q))−1).

A layer mapper 36 maps the modulated symbol streams produced bymodulation mappers 32 onto one or more spatial layers. (For a given setof time and frequency resources allocated to a certain communicationchannel, the multiple transmit and receive antennas add another“spatial” dimension to those resources. One of the possibilities toexploit the additional spatial dimension is by increasing the number ofindependent modulated symbols transmitted per time-frequency resource.The factor of increase, relative to the case of a single transmitantenna and a single receive antenna, is defined as the number ofspatial layers.)

The actual number of spatial layers used by mapper 36 is denotedN_(LAYERS), and is a selectable parameter. The choice of this value maydepend, for example, on the channel conditions between transmitter 20and a given receiver to which the transmission is intended. Each spatiallayer comprises a stream of complex values, which are to be subsequentlytransmitted over the MIMO communication channel. In some embodiments,transmitter 20 sets an upper limit on the value of N_(LAYERS), as willbe discussed in detail further below. Several suitable mapping schemesthat can be used by layer mapper 36 are shown in FIG. 4A-7B below.

The mapped spatial layers are provided to a precoder 40. Precoder 40maps the N_(LAYERS) spatial layers onto N_(TX) transmission channels,corresponding to N_(TX) antenna ports 52 of the transmitter. (Note thata given antenna port may not necessarily correspond to a single physicalantenna, but may correspond to a “virtual antenna” whose transmittedsignal is generated—in a manner that the receiver need not necessarilybe aware of—as a superposition (a weighted sum) of the signals stemmingfrom a number of physical antennas. Note also that the number of antennaports may be larger than the number of layers.) Resource mappers 44allocate resource elements (time-frequency allocations) to therespective transmission channels. The outputs of mappers 44 areprocessed by respective Orthogonal Frequency Division Multiplexing(OFDM) generators 48, which produce OFDM signals that are transmittedvia antenna ports 52 toward the receiver.

Transmitter 20 comprises a controller 56, which configures and controlsthe different transmitter elements. In particular, controller 56comprises a layer and code word setting module 60, which sets the numberof spatial layers and the number of code words to be used by thetransmitter. The functions of module 60 are explained in detail below.

The transmitter configuration shown in FIG. 1 is a simplified exampleconfiguration, which is depicted for the sake of conceptual clarity. Inalternative embodiments, any other suitable transmitter configurationcan also be used. For example, although the embodiments described hereinrefer mainly to transmitters having eight transmit antenna ports, themethods and systems described herein can be used with any other suitablenumber of antenna ports. Transmitter elements that are not mandatory forexplanation of the disclosed techniques, such as various Radio Frequency(RF) elements, have been omitted from FIG. 1 for the sake of clarity.

The different components of transmitter 20 may be implemented usingdedicated hardware, such as using one or more Application-SpecificIntegrated Circuits (ASICs) and/or Field-Programmable Gate Arrays(FPGAs). Alternatively, some transmitter components may be implementedusing software running on general-purpose hardware, or using acombination of hardware and software elements. Typically, controller 56comprises a general-purpose processor, which is programmed in softwareto carry out the functions described herein, although it too may beimplemented on dedicated hardware. The software may be downloaded to theprocessor in electronic form, over a network, for example, or it may,alternatively or additionally, be provided and/or stored on tangiblemedia, such as magnetic, optical, or electronic memory.

When transmitter 20 transmits to a given receiver (not shown in thefigures), module 60 typically sets the actual number of spatial layers(N_(LAYERS)) according to the channel conditions between the transmitterand the receiver. In accordance with an embodiment, the maximum possiblenumber of layers is given by min (N_(TX), N_(RX)), wherein N_(TX) andN_(RX) denote the number of transmit and receive antenna ports,respectively. Module 60 typically selects the actual number of layersadaptively, in an attempt to maximize the data throughput that can betransferred reliably to the receiver over the present channel (andnoise-plus-interference).

The number of layers that is predicted to yield the maximal throughputis sometimes called the “MIMO channel rank.” In some cases (e.g., whenthe transmitter lacks reliable information as to the channel conditions)the receiver notifies the transmitter as to the “preferred rank” (e.g.,a rank indicator, in the terminology of section 7 of the 3GPP TS 36.213specification, cited above). In some embodiments, module sets the actualnumber of layers based on the preferred rank that is fed-back from thereceiver. The statistics of which ranks are preferable in scenarios ofvarying (e.g. fading) channels typically depends on the amount ofcorrelation among the different communication channels between thetransmitter and the receiver (i.e., between the different transmit andreceive antenna pairs) and on the Signal-to-Noise Ratio (SNR) at thereceiver. Lower ranks are more likely to be preferable at low SNR and/orin highly-correlated channels, and vice versa.

When the communication channels exhibit little correlation and/orprovide a relatively high SNR, the receiver is more likely to succeeddecoding a large number of spatial layers. In such cases, in accordancewith an embodiment of the invention, module 60 typically sets arelatively large number of layers, so as to provide a relatively highdata throughput. However, when the communication channels are highlycorrelated and/or lead to a relatively low SNR, the receiver is onlylikely to succeed decoding a smaller number of spatial layers. In suchcases, module 60 may reduce the number of actual layers accordingly.

In practice, however, the likelihood of reaching and exploiting thetheoretical maximum number of layers min (N_(TX), N_(RX)) is very low.In most practical scenarios, at least some correlation exists among themultiple communication channels, and the situation where the maximumnumber of layers is beneficial—namely, a very high SNR—is seldomreached.

Therefore, in some embodiments, module 60 in transmitter 20 sets anupper limit, which is less than the above-mentioned theoretical limit,on the number of spatial layers. Typically, this upper limit is appliedto the transmitter operation as a whole, irrespective of any givenreceiver. The upper limit is therefore expressed in terms of the numberof transmit antenna ports. In other words, transmitter 20 may limitN_(LAYERS) to a value that is less than (and not equal to) N_(TX). Theupper limit is denoted N_(MAX) herein. For example, in an LTE-A eNodeBhaving eight transmit antenna ports (N_(TX)=8), module 60 may set theupper limit to N_(MAX)=4, N_(MAX)=5. N_(MAX)=6 or N_(MAX)=7.Alternatively, any other suitable values of N_(TX) and N_(MAX) can alsobe used.

For a given value of N_(TX), the choice of N_(MAX) trades-offtransmitter complexity and performance. Higher N_(MAX) corresponds to apotentially-higher maximum throughput, but on the other hand means thatthe transmitter is required to store larger mapping tables and supportsimultaneous processing (e.g., mapping and precoding) of a higher numberof symbol streams. Lower N_(MAX) simplifies the transmitter at theexpense of potentially-lower maximum throughput. Since the likelihood ofapproaching the maximum throughput in real-life scenarios is low, asexplained above, limiting N_(MAX) to values less than the number oftransmit antenna ports is often a preferable trade-off.

Moreover, limiting the number of layers enables reduction of signalingresources, which are used for signaling the actual number of layersbetween the transmitter and the receiver (and/or signaling the preferrednumber of layers as feedback from the receiver to the transmitter, asnoted above). For example, if the maximum number of layers is reducedfrom eight to four (i.e., N_(LAYERS)≦N_(MAX)=4), the transmitter canreport the value of N_(LAYERS) to the receiver using only two bitsinstead of three. If N_(MAX) is set to six (N_(LAYERS)≦N_(MAX)=6), threesignaling bits are still needed, but only six out of the eight possiblebit value combinations are used for signaling N_(LAYERS). The remainingtwo bit value combinations are free, and can be reserved for any othersuitable purpose. Thus, in some embodiments, transmitter signalsN_(LAYERS) to the receiver using a signaling protocol in which thesignaling resources (e.g., a field or other data structure), which areallocated to signaling the actual number of layers, are insufficient forsignaling values higher than N_(MAX).

As noted above, in some embodiments the receiver notifies thetransmitter of the preferred number of layers to be used (e.g., using arank indicator field). In these embodiments, the protocol used forsignaling the preferred number of layers to the transmitter may bedefined so that signaling resources (e.g., a field or other datastructure), which are allocated to signaling the preferred number oflayers, are insufficient for signaling values higher than N_(MAX).

FIG. 2 is a flow chart that schematically illustrates a method fortransmission via multiple antennas, in accordance with an embodiment ofthe present invention. In the present example, transmitter 20 isdisposed in an LTE-A base station (eNodeB), which may communicate withmultiple UEs. The method begins with transmitter 20 setting an upperlimit (N_(MAX)) on the number of spatial layers to be used for downlinktransmission, at a limiting step 70. The upper limit is less than thenumber of transmit antenna ports of transmitter 20, i.e.,N_(MAX)<N_(TX).

The eNodeB, and in particular transmitter 20, establishes communicationwith a given UE, at a communication set-up step 74. Based on the channelconditions between the eNodeB and this UE, module 60 in transmitter 20selects an actual number of spatial layers (N_(LAYERS)) for downlinkcommunication with the given UE, at an actual layer selection step 78.Module 60 selects an actual number that does not exceed the upper limitset at step 70 above, i.e., N_(LAYERS)≦N_(MAX). (Additionally oralternatively, the actual number of layers may be set based onclosed-loop channel condition information that is fed-back from thereceiver and/or on a-priori determination of channel conditioninformation. The actual number of layers may be different forclosed-loop and open-loop operation. These features are described indetail further below.)

The transmitter communicates with the given UE using the selected actualnumber of layers. ECC encoders 24 encode the data for transmission so asto produce N_(CW) ECC code words, at an ECC encoding step 82. Scramblers28 scramble the bits of each code word, and modulation mappers 32modulate the scrambled bits to produce streams of encoded symbols, at amodulation step 86. The output of step 86 is a set of N_(CW)≧1 streamsof modulated symbols. Layer mapper 36 maps the N_(CW) streams ofmodulated symbols onto the N_(LAYERS) spatial layers, at a layer mappingstep 90. Any suitable mapping scheme can be used, such as theillustrative mapping schemes described in FIGS. 4A-7B below.

Precoder 40 maps the N_(LAYERS) spatial layers onto the N_(TX) transmitantenna ports 52 of transmitter 20, at a precoding step 94. Thetransmitter transmits the precoded spatial layers via the transmitantenna ports to the given UE, at a transmission step 98.

In some embodiments, module 60 selectively limits the maximum number ofcode words per transmission (N_(CW)) to be less than the actual numberof layers. For example, module 60 may limit the value of N_(CW) to nomore than 2 (i.e., N_(CW)ε{1,2}). Reducing the number of code words pertransmission simplifies the transmitter and reduces the signalingresources needed for signaling the selected N_(CW) value (and/or otherinformation that is signaled per code word) to the receiver. On theother hand, a lower N_(CW) value may somewhat degrade the receiverperformance, for example in receivers that use Sequential InterferenceCancellation (SIC) techniques. Nevertheless, in most cases a maximum ofN_(CW)=2 provides good receiver performance, even for N_(LAYERS)=8.Increasing N_(CW) beyond this value may not provide additionalperformance that justifies the associated complexity. The layer mappingexamples given in FIGS. 4A-7B below demonstrate mapping of one or twocode words onto up to eight spatial layers.

In some embodiments, transmitter 20 sets different upper limits on thenumber of layers, depending on the type of precoding operation appliedby precoder 40. Precoder 40 may apply closed-loop or open-loopprecoding. For example, closed- and open-loop precoding in E-UTRAsystems are described in sections 6.3.4.2.1 and 6.3.4.2.2 of 3GPP TS36.211, cited above, and in section 7 of 3GPP TS 36.213, cited above. Inclosed-loop precoding, the mapping of spatial layers to antenna ports isadaptive based on feedback provided by the receiver. For example, insome embodiments the transmitter and receiver support a predefined set(a “codebook”) of precoding schemes, usually expressed as precodingmatrices. The receiver notifies the transmitter which precoding schemeis preferable at a given point in time, and the transmitter selects andapplies the mapping scheme requested by the receiver. In open-loopprecoding, the transmitter applies a certain precoding schemeirrespective of feedback from the receiver.

When the transmitter uses closed-loop precoding, link adaptation maytrack the channel conditions relatively accurately, and the receiver maybetter exploit the potential spatial multiplexing gain of the multiplechannels. When using open-loop precoding, on the other hand, the actualspatial multiplexing gain is likely to be lower. Therefore, using alarge number of spatial layers is more likely to produce highperformance under closed-loop precoding than under open-loop precoding.Thus, in some embodiments, transmitter 20 sets a certain upper limit(denoted N_(MAX) _(—) _(OL)) on the number of layers when usingopen-loop precoding, and another upper limit (denoted N_(MAX) _(—)_(CL)) on the number of layers when using closed-loop precoding, whereinN_(MAX) _(—) _(OL)<N_(MAX) _(—) _(CL).

FIG. 3 is a flow chart that schematically illustrates a method fortransmission via multiple antennas, in accordance with an embodiment ofthe present invention. The method of FIG. 3 begins with transmitter 20establishing communication with a given UE, at a communicationestablishing step 100. Controller 56 in transmitter 20 checks whetheropen- or closed-loop precoding is used with this UE, at a precoding modechecking step 104. If open-loop precoding is used, module 60 setsN_(MAX)=N_(MAX) _(—) _(OL), at an open-loop layer limiting step 108.Otherwise, i.e., if closed-loop precoding is used, module 60 setsN_(MAX)=N_(MAX) _(—) _(CL), at a closed-loop layer limiting step 112. Asnoted above, N_(MAX) _(—) _(OL)<_(MAX) _(—) _(CL).

Having selectably limited the number of spatial layers to either N_(MAX)_(—) _(OL) or N_(MAX) _(—) _(CL) depending on the precoding mode,transmitter 20 sets the actual number of layers N_(LAYERS) to valuesthat do not exceed the applicable upper limit, at a layer setting step116. From this stage, the method continues similarly to steps 82-98 ofthe method of FIG. 2 above.

In some embodiments, N_(LAYERS) is limited to be less than N_(TX) onlywhen the number of antenna ports is high, and N_(LAYERS)=N_(TX) isallowed below a certain number of antenna ports. This technique can beused to maintain backward compatibility with conventional schemes, e.g.,with LTE systems conforming to the TS 36.211 and 36.213 specifications,cited above. For example, the constraints on the number of layers can beset to 1≦N_(LAYERS)≦min(N_(TX),N_(RX)) if min(N_(TX),N_(RX))≦4, to1≦N_(LAYERS)≦N_(MAX) _(—) _(CL)<min(N_(TX),N_(RX)) ifmin(N_(TX),N_(RX))>4 and closed-loop precoding is used, and to1≦N_(LAYERS)≦N_(MAX) _(—) _(OL)<min(N_(TX),N_(RX)) ifmin(N_(TX),N_(RX))>4 and open-loop precoding is used.

FIGS. 4A-7B are tables showing mapping examples of code words to spatiallayers, in accordance with embodiments of the present invention. FIGS.4A and 4B show one mapping example, FIGS. 5A and 5B show a secondexample, FIGS. 6A and 6B show a third example, and FIGS. 7A and 7B showa fourth example. All four examples refer to an LTE-A eNodeB havingeight antenna ports. These four examples are in no way limiting. Themethods and systems described herein may use any other suitable mappingscheme.

In a given example, each row defines the mapping of a certain number ofcode words (N_(CW)) to a certain number of spatial layers (N_(LAYERS)).In the examples, d^((q))(n) denotes the n′th modulated symboloriginating from code word q. x^((p))(n) denotes the n′th complex valueof the p′th spatial layer. As can be seen in the examples, in some casesthe symbols of a given code word are de-multiplexed over two or morelayers. In other cases, the symbols of a given code word are mapped to asingle layer. The examples given herein attempt to distribute thesymbols among the layers evenly, although this feature is notnecessarily mandatory.

In all four examples, the mapping of code words to layers forN_(LAYERS)≦4 conforms to the mapping specified in section 6.3.3.2 of the3GPP TS 36.211 specification, cited above. This feature maintainsbackward compatibility, i.e., enables the eNodeB to communicate withLTE-compliant UEs. This feature is, however, by no means mandatory.Other mapping schemes may differ from the 3GPP TS 36.211 specificationas desired.

In all four examples, the maximum number of code words is two. As notedabove, increasing the number of code words beyond two usually does notprovide significant performance improvement. Nevertheless, inalternative embodiments, the mapping may specify higher numbers of codewords, as well.

As explained above, transmitter 20 may selectably set an upper limit onthe number of layers, which is less than the number of antenna ports.The transmitter may set this upper limit, for example, by storing and/orusing only a subset of the rows of a given mapping table. For example,when setting N_(MAX)=6, the transmitter may omit the rows correspondingto N_(LAYERS)>6. This technique may simplify the design of layer mapper36 and reduce the memory space used for storing the mapping table.

Additionally or alternatively, the transmitter may omit one or more ofthe rows of the mapping table in order to reduce memory requirements,computational complexity and signaling resources. The omitted rows donot necessarily correspond to large numbers of layers. For example, thetransmitter may omit the odd-order rows or even-order rows of themapping table.

Although the embodiments described herein mainly address setting thenumber of spatial layers in LTE-A transmitters, the methods and systemsdescribed herein can also be used in other applications, such as in IEEE802.16 transceivers.

It will thus be appreciated that the embodiments described above arecited by way of example, and that the present invention is not limitedto what has been particularly shown and described hereinabove. Rather,the scope of the present invention includes both combinations andsub-combinations of the various features described hereinabove, as wellas variations and modifications thereof which would occur to personsskilled in the art upon reading the foregoing description and which arenot disclosed in the prior art.

The invention claimed is:
 1. A method for communication, comprising: ina transmitter having a first number of transmit antenna ports, settingan upper limit on a second number of spatial layers to be used by thetransmitter to be less than the first number; allocating an actualnumber of the spatial layers, which does not exceed the upper limit, fortransmission to a given receiver; encoding input data with an ErrorCorrection Code (ECC) to produce a given number of code words that isrestricted to be at most two, and modulating the code words to produce arespective given number of streams of modulated symbols; mapping thestreams of the modulated symbols onto the allocated actual number of thespatial layers; and transmitting the actual number of the spatial layersfrom the transmitter to the given receiver, and comprising, when theactual number of spatial layers is between one and four, mapping themodulated symbols onto the spatial layers in accordance with the tableshown in FIG. 4A in which d^((q))(n) denotes an n^(th) modulated symboloriginating from a code word q, and x^((p))(n) denotes an n^(th) valueof a spatial layer p.
 2. A method for communication, comprising: in atransmitter having a first number of transmit antenna ports, setting anupper limit on a second number of spatial layers to be used by thetransmitter to be less than the first number; allocating an actualnumber of the spatial layers, which does not exceed the upper limit, fortransmission to a given receiver; encoding input data with an ErrorCorrection Code (ECC) to produce a given number of code words that isrestricted to be at most two, and modulating the code words to produce arespective given number of streams of modulated symbols; mapping thestreams of the modulated symbols onto the allocated actual number of thespatial layers; and transmitting the actual number of the spatial layersfrom the transmitter to the given receiver, and comprising, when theactual number of spatial layers is between five and eight, mapping themodulated symbols onto the layers in accordance with the table shown inFIGS. 4A and 4B in which d^((q))(n) denotes an n^(th) modulated symboloriginating from a code word q, and x^((p))(n) denotes an n^(th) valueof a spatial layer p.
 3. The method according to claim 2, andcomprising, when the actual number of spatial layers is between one andfour, mapping the modulated symbols onto the spatial layers inaccordance with the table shown in FIG. 4A in which d^((q))(n) denotesan n^(th) modulated symbol originating from a code word q, andx^((p))(n) denotes an n^(th) value of a spatial layer p.
 4. A method forcommunication, comprising: in a transmitter having a first number oftransmit antenna ports, setting an upper limit on a second number ofspatial layers to be used by the transmitter to be less than the firstnumber; allocating an actual number of the spatial layers, which doesnot exceed the upper limit, for transmission to a given receiver;encoding input data with an Error Correction Code (ECC) to produce agiven number of code words that is restricted to be at most two, andmodulating the code words to produce a respective given number ofstreams of modulated symbols; mapping the streams of the modulatedsymbols onto the allocated actual number of the spatial layers; andtransmitting the actual number of the spatial layers from thetransmitter to the given receiver, and comprising, when the actualnumber of spatial layers is between five and eight, mapping themodulated symbols onto the layers in accordance with the table shown inFIGS. 5A and 5B in which d^((q))(n) denotes an n^(th) modulated symboloriginating from a code word q, and x^((p))(n) denotes an n^(th) valueof a spatial layer p.
 5. The method according to claim 4, andcomprising, when the actual number of spatial layers is between one andfour, mapping the modulated symbols onto the spatial layers inaccordance with the table shown in FIG. 4A in which d^((q))(n) denotesan n^(th) modulated symbol originating from a code word q, andx^((p))(n) denotes an n^(th) value of a spatial layer p.
 6. A method forcommunication, comprising: in a transmitter having a first number oftransmit antenna ports, setting an upper limit on a second number ofspatial layers to be used by the transmitter to be less than the firstnumber; allocating an actual number of the spatial layers, which doesnot exceed the upper limit, for transmission to a given receiver;encoding input data with an Error Correction Code (ECC) to produce agiven number of code words that is restricted to be at most two, andmodulating the code words to produce a respective given number ofstreams of modulated symbols; mapping the streams of the modulatedsymbols onto the allocated actual number of the spatial layers; andtransmitting the actual number of the spatial layers from thetransmitter to the given receiver, and comprising, when the actualnumber of spatial layers is between five and eight, mapping themodulated symbols onto the layers in accordance with the table shown inFIGS. 6A and 6B in which d^((q))(n) denotes an n^(th) modulated symboloriginating from a code word q, and x^((p))(n) denotes an n^(th) valueof a spatial layer p.
 7. The method according to claim 6, andcomprising, when the actual number of spatial layers is between one andfour, mapping the modulated symbols onto the spatial layers inaccordance with the table shown in FIG. 4A in which d^((q))(n) denotesan n^(th) modulated symbol originating from a code word q, andx^((p))(n) denotes an n^(th) value of a spatial layer p.
 8. A method forcommunication, comprising: in a transmitter having a first number oftransmit antenna ports, setting an upper limit on a second number ofspatial layers to be used by the transmitter to be less than the firstnumber; allocating an actual number of the spatial layers, which doesnot exceed the upper limit, for transmission to a given receiver;encoding input data with an Error Correction Code (ECC) to produce agiven number of code words that is restricted to be at most two, andmodulating the code words to produce a respective given number ofstreams of modulated symbols; mapping the streams of the modulatedsymbols onto the allocated actual number of the spatial layers; andtransmitting the actual number of the spatial layers from thetransmitter to the given receiver, and comprising, when the actualnumber of spatial layers is between five and eight, mapping themodulated symbols onto the layers in accordance with the table shown inFIGS. 7A and 7B in which d^((q))(n) denotes an n^(th) modulated symboloriginating from a code word q, and x^((p))(n) denotes an n^(th) valueof a spatial layer p.
 9. The method according to claim 8, andcomprising, when the actual number of spatial layers is between one andfour, mapping the modulated symbols onto the spatial layers inaccordance with the table shown in FIG. 4A in which d^((q))(n) denotesan n^(th) modulated symbol originating from a code word q, andx^((p))(n) denotes an n^(th) value of a spatial layer p.
 10. Acommunication apparatus, comprising: a first number of transmit antennaports; and a transmitter, which is configured to set an upper limit on asecond number of spatial layers to be used by the transmitter to be lessthan the first number, to allocate an actual number of the spatiallayers, which does not exceed the upper limit, for transmission to agiven receiver, to encode input data with an Error Correction Code (ECC)to produce a given number of code words that is restricted to be at mosttwo, to modulate the code words to produce a respective given number ofstreams of modulated symbols, to map the streams of the modulatedsymbols onto the allocated actual number of the spatial layers, totransmit the actual number of the spatial layers to the given receiver,and, when the actual number of spatial layers is between one and four,to map the modulated symbols onto the spatial layers in accordance withthe table shown in FIG. 4A in which d^((q))(n) denotes an n^(th)modulated symbol originating from a code word q, and x^((p))(n) denotesan n^(th) value of a spatial layer p.
 11. A communication apparatus,comprising: a first number of transmit antenna ports; and a transmitter,which is configured to set an upper limit on a second number of spatiallayers to be used by the transmitter to be less than the first number,to allocate an actual number of the spatial layers, which does notexceed the upper limit, for transmission to a given receiver, to encodeinput data with an Error Correction Code (ECC) to produce a given numberof code words that is restricted to be at most two, to modulate the codewords to produce a respective given number of streams of modulatedsymbols, to map the streams of the modulated symbols onto the allocatedactual number of the spatial layers, to transmit the actual number ofthe spatial layers to the given receiver, and, when the actual number ofspatial layers is between five and eight, to map the modulated symbolsonto the layers in accordance with the table shown in FIGS. 4A and 4B inwhich d^((q))(n) denotes an n^(th) modulated symbol originating from acode word q, and x^((p))(n) denotes an n^(th) value of a spatial layerp.
 12. The apparatus according to claim 11, wherein the transmitter isconfigured, when the actual number of spatial layers is between one andfour, to map the modulated symbols onto the spatial layers in accordancewith the table shown in FIG. 4A in which d^((q))(n) denotes an n^(th)modulated symbol originating from a code word q, and x^((p))(n) denotesan n^(th) value of a spatial layer p.
 13. A communication apparatus,comprising: a first number of transmit antenna ports; and a transmitter,which is configured to set an upper limit on a second number of spatiallayers to be used by the transmitter to be less than the first number,to allocate an actual number of the spatial layers, which does notexceed the upper limit, for transmission to a given receiver, to encodeinput data with an Error Correction Code (ECC) to produce a given numberof code words that is restricted to be at most two, to modulate the codewords to produce a respective given number of streams of modulatedsymbols, to map the streams of the modulated symbols onto the allocatedactual number of the spatial layers, to transmit the actual number ofthe spatial layers to the given receiver, and, when the actual number ofspatial layers is between five and eight, to map the modulated symbolsonto the layers in accordance with the table shown in FIGS. 5A and 5B inwhich d^((q))(n) denotes an n^(th) modulated symbol originating from acode word q, and x^((p))(n) denotes an n^(th) value of a spatial layerp.
 14. The apparatus according to claim 13, wherein the transmitter isconfigured, when the actual number of spatial layers is between one andfour, to map the modulated symbols onto the spatial layers in accordancewith the table shown in FIG. 4A in which d^((q))(n) denotes an n^(th)modulated symbol originating from a code word q, and x^((p))(n) denotesan n^(th) value of a spatial layer p.
 15. A communication apparatus,comprising: a first number of transmit antenna ports; and a transmitter,which is configured to set an upper limit on a second number of spatiallayers to be used by the transmitter to be less than the first number,to allocate an actual number of the spatial layers, which does notexceed the upper limit, for transmission to a given receiver, to encodeinput data with an Error Correction Code (ECC) to produce a given numberof code words that is restricted to be at most two, to modulate the codewords to produce a respective given number of streams of modulatedsymbols, to map the streams of the modulated symbols onto the allocatedactual number of the spatial layers, to transmit the actual number ofthe spatial layers to the given receiver, and, when the actual number ofspatial layers is between five and eight, to map the modulated symbolsonto the layers in accordance with the table shown in FIGS. 6A and 6B inwhich d^((q))(n) denotes an n^(th) modulated symbol originating from acode word q, and x^((p))(n) denotes an n^(th) value of a spatial layerp.
 16. The apparatus according to claim 15, wherein the transmitter isconfigured, when the actual number of spatial layers is between one andfour, to map the modulated symbols onto the spatial layers in accordancewith the table shown in FIG. 4A in which d^((q))(n) denotes an n^(th)modulated symbol originating from a code word q, and x^((p))(n) denotesan n^(th) value of a spatial layer p.
 17. A communication apparatus,comprising: a first number of transmit antenna ports; and a transmitter,which is configured to set an upper limit on a second number of spatiallayers to be used by the transmitter to be less than the first number,to allocate an actual number of the spatial layers, which does notexceed the upper limit, for transmission to a given receiver, to encodeinput data with an Error Correction Code (ECC) to produce a given numberof code words that is restricted to be at most two, to modulate the codewords to produce a respective given number of streams of modulatedsymbols, to map the streams of the modulated symbols onto the allocatedactual number of the spatial layers, to transmit the actual number ofthe spatial layers to the given receiver, and, when the actual number ofspatial layers is between five and eight, to map the modulated symbolsonto the layers in accordance with the table shown in FIGS. 7A and 7B inwhich d^((q))(n) denotes an n^(th) modulated symbol originating from acode word q, and x^((p))(n) denotes an n^(th) value of a spatial layerp.
 18. The apparatus according to claim 17, wherein the transmitter isconfigured, when the actual number of spatial layers is between one andfour, to map the modulated symbols onto the spatial layers in accordancewith the table shown in FIG. 4A in which d^((q))(n) denotes an n^(th)modulated symbol originating from a code word q, and x^((p))(n) denotesan n^(th) value of a spatial layer p.